These struts / P-brackets are usually bedded and through-bolted with a hefty backing plate on the inside of the hull
. The outside of the hull
may be faired into the strut base. If properly sized and installed, the strut itself should never loosen and never exhibit any noticeable play.
Do you have access to the inside of the hull above the strut? If you can reach it from the inside (often difficult or impossible), you should be able to loosen the bolts and remove the strut. The odds are pretty good that at least a couple of them are loose or corroded.
If it doesn't get fixed... well, sudden catastrophic failure is, indeed, as bad as it sounds. Best case, you'd notice in time to kill the engine
; worst case, the prop could come up through the bottom.